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Microsoft Encarta Excerpt on Cancer: Cancer (medicine), new growth of tissue resulting from the continuous and rapid production of abnormal cells that invade and destroy other tissues. Cancer, which may arise from any type of cell in any body tissue, is not a single disease but includes a large number of diseases classified according to the tissue and type of cell in which new growth occurs. Several hundred such classes exist, constituting three major subtypes:
More than 1,350,000 new cases of cancer occur in the United States each year. Cancer is the second leading cause of death in the nation, accounting for about 550,000 deaths annually, and the leading cause of death from disease in children between the ages of 1 and 14. Worldwide, the incidence of cancer varies enormously among different geographic areas. The death rate from all cancers in males is 311 per 100,000 in Luxembourg (the highest) as compared to 38 in El Salvador (the lowest). For women it is 175 in Denmark and 49 in El Salvador. The figures for the United States are 217 per 100,000 men and 137 per 100,000 women. For particular cancers, the difference between countries may be as high as 40-fold. Evidence from studies of populations that migrated from one geographic area to another suggests that these variations are due to differences in lifestyle rather than ethnic origin. This is consistent with other evidence that most cancers are predominantly related to environmental causes rather than heredity, although the two factors interact. The cancers that cause the most deaths in the United States are lung cancer (first in each sex), colorectal cancer (second in both sexes combined), breast and uterine cancers in women, and prostate cancer in men. Together they account for more than 55 percent of all deaths from cancer. The most frequently occurring cancers are cancers of the skin, with over 800,000 cases of curable cancer per year and nearly 40,000 cases of malignant melanoma per year. Since 1949 cancer mortality in the United States has been higher among men than women. The sex ratios of different cancers vary considerably. Cancer mortality is higher among black people than among white people for reasons not yet fully understood. A cancerous growth, or neoplasm, is clonal—that is, all its cells are descendants of a single cell. These cells escape the control of normal forces that regulate cell growth by using an enzyme called telomerase, which allows malignant cells to reproduce endlessly. Malignant cells are unable to differentiate or mature into an adult, functioning state. As these cells multiply, they may form a mass called a tumor, which enlarges and continues to grow without regard to the function of the tissue in which it originated. Almost all cancers form tumors, but not all tumors are cancerous, or malignant; the greatest number are benign, or non-life-threatening. Benign tumors grow only in the tissue in which they originate and are usually separated from neighboring tissue by a surrounding capsule, or sac. Benign tumors generally grow slowly and in structure closely resemble the tissue in which they first grew. In some cases they may endanger a person’s health by obstructing, compressing, or displacing neighboring tissues or organs, as in the brain. A few benign tumors, such as polyps of the colon, may later become cancerous. The most significant attribute of malignant tumors is their ability to spread beyond the location where they first begin to grow. Cancer may invade or extend into neighboring tissue or travel to distant sites, forming secondary growths known as metastases. The routes and sites of metastases vary with different primary cancers:
Cancer cells, even when widely spread throughout the body, may retain the physical and biological characteristics of the tissue in which they originated. Thus, a pathologist can often determine the site of origin (the tissue where cancer cells first begin to grow) of a metastatic tumor by microscopic examination of the cancerous tissue. Tumors of the endocrine glands, for instance, may produce excessive amounts of the hormone produced by parent tissue. In general, the less closely a cancer resembles the tissue in which it first grew, the more malignant and rapidly invasive it tends to be. Over time, a tumor tends to grow more rapidly and may develop increased resistance to various forms of treatment. Cancer may be caused by a number of factors. These include inheriting a gene that predisposes an individual to a particular type of cancer, contracting a virus that causes a specific form of cancer, or being exposed to certain types of radiation and particular chemicals that cause cancer. Changes in the immune system and exposure to certain environmental factors can also cause cancer. Researchers study how these various factors interact to cause malignancy. Cancer is basically a genetic process. Gene abnormalities can be inherited, or produced in a cell by a virus or by damage from an outside source. A series of mutations that occur in a specific order may eventually lead to a single cell that is malignant and proliferates as a clone. New studies indicate that some individuals are more likely to develop cancer if they lack or have a defective copy of a particular gene, known as p53, that suppresses the growth of abnormal cells. This gene appears to affect cancer-causing activity in several types of tumors, including those of the brain, breast, prostate, and kidney. Another tumor-suppressing gene known as RB may undergo mutations, thereby enabling cells to multiply endlessly and possibly result in cancer. It is estimated that no more than 20 percent of cancers are based on inherited genetic information. Inherited, or familial, breast cancer accounts for 5 to 10 percent of all breast cancers. Mutations of two genes, called BRCA1 and BRCA2, are responsible for about 90 percent of familial breast cancers. Researchers suspect that the two genes may also play a role in breast cancer cases that occur in women with no family history of the disease. Cancer of the colon is more common in families whose members have a history of polyps in the colon. A type of retinoblastoma (a malignant tumor of the retina of the eye) occurs only when a specific gene is absent. A genetic abnormality that weakens the body’s defenses against carcinogens has been linked to prostate cancer. In humans the Epstein-Barr virus causes Burkitt’s lymphoma and lymphoepitheliomas (a malignant tumor that occurs in the nose and throat area); the hepatitis B virus causes hepatocarcinoma (a malignant tumor of the liver); and two types of the human papilloma virus cause malignant tumors of the cervix. These viruses use deoxyribonucleic acid (DNA)—the genetic material of a cell—to make copies of themselves. Other viruses such as the HTLV virus that produces T-cell leukemia use ribonucleic acid (RNA)—the genetic material of certain viruses—to reproduce. These viruses are known as retroviruses. In the presence of an enzyme called reverse transcriptase, they induce an infected cell to make DNA copies of the virus’s genes, which can then be incorporated into the cell’s DNA. Such viruses may contain a gene, called a viral oncogene, capable of transforming normal cells into malignant cells. Research indicates that each viral oncogene has a counterpart in the normal human cell. This counterpart, called a proto-oncogene, or cellular oncogene, produces proteins that encourage a cell to enlarge and divide. When a proto-oncogene mutates into a carcinogenic oncogene, it may produce an abnormally large amount of these growth proteins, resulting in excessive cell division. Radiation is a potent cause of cancer. Radiation causes changes in DNA, including breaks in the chromosomes (the tiny structures in cells that contain DNA) and chromosome transpositions—the exchange of chromosomal material between two chromosomes. Radiation causes a normal cell to slowly become cancerous over a period of years. During this period, the cell may be more susceptible to other cancer-causing factors. The most common sources of radiation include the ultraviolet B rays of the sun, which cause over 90 percent of all skin cancers, and radon gas, which is emitted from the earth and seeps into buildings—sometimes collecting in high levels. Breathing high levels of radon gas for long periods of time may cause lung cancer. The process by which chemical agents cause cancer has been extensively studied. Some chemicals act as initiators—that is, they start the cancer process after only a single exposure. Cancer may not appear until after a long period and may require exposure to another agent that acts as a promoter—a substance that encourages the cancer to grow. Tobacco smoke contains both chemical initiators and promoters, and alcohol contains promoters. For smokers, chronic abuse of alcohol greatly increases the risk of lung cancer (see Carcinogen). The body’s immune system can recognize malignant cells and stimulate the production of cells that are often able to destroy the cancer. If the immune system is weakened, however, the body may be unable to destroy the malignant cells and cancer may result. Various diseases can severely weaken the immune system, including see acquired immune deficiency syndrome (AIDS), and inherited immune deficiency diseases. Immunosuppressive drugs, used to prevent the body from rejecting an organ after an organ transplant, also impair the immune system’s ability to protect the body from cancer. About 80 percent of cancers may be caused by environmental factors. Up to 60 percent of cancer deaths in the United States are due to tobacco smoke (both firsthand and secondhand smoke) and diet. Smoking is responsible for about half of these deaths while inadequate diet accounts for the remaining portion. Obesity is also a risk factor for a number of cancers, especially cancers of the breast, colon, uterus, and prostate. The common component that unites the factors that cause cancer may be the viral oncogene, which causes normal cells to mutate into cancerous cells. Research has shown a malignancy may be the result of a series of mishaps beginning with an abnormal gene or a somatic mutation (a mutation of a normal body tissue cell) followed by a promoting activity that stimulates oncogenes to produce proteins that encourage growth. The stimulation of these growth proteins then causes the abnormal cell to rapidly multiply, and a defect in the immune system permits the abnormal cells to escape destruction. The earlier a cancer is diagnosed and treated, the greater the chance for a cure. The regular screening of apparently healthy people permits diagnosis before development of symptoms, when the cancer is most curable. The cancers for which excellent or reasonably good screening is available are among the major killers: breast, colon and rectum, cervix, and prostate cancer. Early diagnosis of cancers not susceptible to screening depends upon recognition of early signs of disease by the patient. There are seven classic danger signals of cancer:
A diagnosis of cancer begins with a thorough history and physical examination, including inspection, both visually and manually, of all accessible areas of the body, especially the skin, neck, breasts, abdomen, testicles, and the areas that contain lymph nodes. It specifically includes examination of body openings, particularly rectal examination for cancers of the rectum or prostate and pelvic examination for cancers of the cervix or uterus. Over the past several years, a variety of new diagnostic tests have been developed to detect the presence of cancer using only a small sample of blood or urine. These include tests for prostate and bladder cancer. Tests that identify genes believed to cause certain cancers—such as breast cancer—are also now available. There is considerable debate over whether these tests help doctors detect cancers earlier or simply alarm currently healthy individuals with knowledge of potential medical problems in the future. For some people, however, test results may enable them to alter their lifestyles and minimize exposure to factors that may trigger cancer. The biopsy remains the only definitive method for the diagnosis of a cancer. In a biopsy, a section of tissue is removed from the original tumor or from a metastasis. Modern technology has greatly reduced the need for open surgical biopsy. Guided by touch or medical imaging techniques that view internal areas of the body, a physician can biopsy a tumor in almost any part of the body using a thin, flexible needle. This minimally invasive technique permits diagnosis before surgery so the doctor and patient can make more informed plans about treatment and surgery, if required. Once a cancer has been diagnosed, the extent, or stage, of disease must be evaluated. Chances for recovery and appropriate treatment vary with the stage of disease. Four stages have been defined, each stage being progressively more serious. Stage one tumors are small and local, stage two and stage three tumors are larger and usually involve lymph nodes, and stage four tumors have metastasized to other areas of the body. The traditional methods for treating cancer have been surgery, radiation, and chemotherapy. More recent treatment options include hormone therapy, combined modality therapy (in which several types of treatment are used together), and biologic therapies such as immunotherapy, which triggers the body’s immune system to attack cancerous cells. Gene therapy shows promise as an effective future cancer treatment. The principal approach to curing cancer is to remove all malignant cells by a surgical operation. In the past this meant the removal of all involved tissue and as much potentially involved tissue as possible, including adjacent tissues and lymph nodes. For some tumors, notably cancer of the breast, radical surgery like see mastectomy is not always necessary. Refinements in surgical techniques, improved knowledge of physiology, advances in anesthesiology, readily available blood products, and potent antibiotics have permitted less extensive surgery with more rapid recovery and less disability. Many cancers, though, are too advanced at the time of diagnosis to permit cure by surgery. If the cancer extends into neighboring tissues that cannot be removed or if distant metastases are already present, surgery will not cure the cancer. Even when it is clear that a surgical cure is impossible, however, surgery may help relieve symptoms or reduce the tumor’s size to improve the response to subsequent radiation or chemotherapy. Radiation therapy uses powerful x-rays (produced when a beam of electrons strikes a heavy-metal target) or gamma rays (emitted by radioactive decay) to destroy cancerous tissue. Tumors easily reached with a source of radiation—either a beam of radiation or through tiny radioactive implants—may be cured by radiation therapy. Radiation therapy is useful when a tumor is located where it cannot be surgically removed because surgery would damage vital adjacent tissue or because a tumor has begun to grow in adjacent tissues or organs that cannot be removed. Radiation therapy is also used to reduce the effects of cancer, especially of metastatic tumors. Radiation can also be used before surgery to sterilize tumor cells and prevent them from spreading to other parts of the body during surgery. Radiation may shrink the tumor and make surgery easier, or shrink an inoperable tumor to an operable size. For other tumors radiation may be used following surgery. New techniques in radiation therapy now allow tumors to be precisely targeted by a beam of radiation, eliminating damage to healthy surrounding tissues. These techniques are known as conformal radiotherapy because the radiation beam conforms to the shape of the tumor. Chemotherapy is the use of drugs in the treatment of cancer. Since a drug is distributed throughout the body by the bloodstream, chemotherapy is useful for tumors that have spread beyond the area accessible by surgery or radiation therapy. Of the many anticancer drugs used, most work by interfering with a cell’s ability to replicate DNA. Therefore, rapidly dividing cells are the most sensitive to chemotherapy. Tumors have the highest proportion of these dividing cells, but some healthy tissues, such the bone marrow and the lining of the gastrointestinal tract, also contain many of these cells. These are the areas of the body most sensitive to the toxic effects of chemotherapy, thus the therapy’s side effects often involve them. For example, many patients who undergo chemotherapy experience anemia, internal bleeding, reduced immune system protection, diarrhea, nausea, and vomiting. The dosage of an anticancer drug taken to treat the disease is limited primarily by the severity of these side effects involving the gastrointestinal tract and bone marrow. Normal bone marrow cells can divide faster than malignant cells and thus recover from chemotherapy more rapidly. This quick recovery permits the administration of a repeat cycle of an anticancer drug before the tumor has regrown. Repeated cycles can steadily deplete a tumor before it becomes resistant. Some tumors are so sensitive to chemotherapy that a chemotherapeutic cure is possible in a high percentage of cancer cases. These cases include uterine cancer, acute leukemia (especially in children), Hodgkin’s disease and diffuse large-cell lymphoma, testicular carcinoma, ovarian carcinoma, small-cell carcinoma of the lung, and several cancers in children. These cancers often have already spread at the time of diagnosis and cannot be treated by other means. Other advanced cancers respond well to chemotherapy and can be controlled for long periods. Two major problems that limit the usefulness of chemotherapy are the toxic effects of the drugs on normal cells and the resistance of the cancerous cells to the drugs administered. Techniques that avoid or control toxicity and reduce the risk of resistance have steadily improved. It is important to begin treatment as early as possible, to use the optimal dose of the drug, and repeat cycles as quickly as possible while giving the patient a chance to recover from toxicity. New drugs that prevent vomiting are now available, and scientists are developing drugs that stimulate bone marrow cells and the cells that line the gastrointestinal tract to regenerate more quickly after chemotherapy. The use of multiple drugs to treat cancer has also proven effective, particularly in stage two and stage three cancers. Combination chemotherapy employs several drugs (often three to six at a time), each of which is effective as a single agent. Each of the drugs works against the cancerous cells differently, so it is unlikely that the tumor can develop resistance against all of them. Chemotherapy may be used with surgery or radiation in combined modality therapy. It is often used as an adjuvant—that is, to increase the effect of the treatment—when surgery is the primary therapy. As such it is usually given after surgery. The major purpose of chemotherapy as an adjuvant is to kill off micrometastases that may have been established in other parts of the body before surgery. This type of therapy has greatly increased the cure rate of breast cancer. In another form of combined modality therapy called induction chemotherapy the drug treatment is given before surgery or radiation. This type of treatment destroys any micrometastases as soon as possible and may also shrink a tumor to a more easily operable size. Another treatment is concomitant chemoradiotherapy, which involves using chemotherapy at the same time as surgery or radiation therapy. This treatment is most effective against tumors that have metastasized or would recur after surgery or radiation therapy alone. Many cancers that originate in hormone-dependent tissues such as the breast, the prostate, the endometrium, and the thyroid respond to manipulation of these hormones. This treatment may consist of removing the source of the stimulating hormone to stop the cancerous cells from functioning, or administering other hormones and antihormones to counter or block the cells’ ability to function. Hormone therapy is effective only in individuals with tumors present in or derived from hormonal tissues. Several promising new approaches to the cancer treatment are currently being investigated. One area of research involves a new drug called endostatin that shrinks tumors by suppressing their ability to produce capillaries—the tiny branching vessels that supply tissues with the blood needed for growth. In tests on mice, endostatin has reduced marble-sized tumors to microscopic size. Other research focuses on biological agents known as biological response modifiers, used to alter the body’s response to cancer. Another approach involves biological agents that stimulate certain cells to attack malignant cells. Interleukin-2, used to stimulate a patient’s lymphokine-activated killer lymphocytes (LAK cells), a type of immune cell, is one example of this agent. Researchers are also experimenting with developing tumor-specific antigens that prompt the immune system to produce antibodies. These antitumor antibodies may then be used to treat cancer either directly or by attaching antibodies to a chemotherapeutic agent. The antibodies would identify the malignant cell and stick to it, thus delivering the drug directly to the target cell. Gene therapy research involves inserting into a tumor a type of gene that directs cells to stop dividing or commit suicide. Early results indicate that in some tumors, growth can be at least temporarily halted or even reversed. Some researchers are now combining gene therapy with radiation therapy. The genes inserted into tumors do not function until they are exposed to radiation. Still other research involves manipulating the genes of a virus known as an adenovirus. In one scenario, a gene useful for treatment of cancer is spliced into the adenovirus. The virus is then allowed to enter the body, delivering the useful gene to all cells it invades, including the cancerous cells it is targeted to treat. In other experiments with adenoviruses, a specific gene that allows the virus to reproduce in normal cells is removed and the altered adenovirus is allowed to invade the cells of the body. Because of the missing gene, the virus does not affect normal cells. When it enters malignant cells, however, it is able to multiply and eventually kill the cancer. For decades in the United States the number of deaths from cancer has risen rapidly and steadily, from 298,000 in 1965 to more than 550,000 in 1996. However, these figures are affected by increasing population and the growing percentage of older adults, who traditionally have a higher incidence of cancer. The figures also reflect high numbers of people who smoke tobacco (see Smoking). As a result, the incidence of lung cancer, the most common cause of death from cancer, has continued to rise. Excluding lung cancer, the death rate from cancer has leveled off. Other cancers have a rising incidence, but for many improvement in the cure rate exceeds the increase in incidence. The treatment of cancer has improved remarkably since 1940, when less than one in four Americans with cancer survived more than five years. In 1996, 40 percent of Americans with cancer will survive that long. It is estimated that more than 10 million living Americans have had cancer. Seven million have survived more than five years, and nearly all of the survivors are regarded as cured. The death rate from cancer has fallen progressively for all age groups below 55 years old. This drop can be attributed to more healthful eating habits, a decline in smoking and alcohol use, a less-contaminated environment with decreased likelihood of exposure to carcinogens, and earlier diagnosis of treatable cancers. Skin cancers (most of which are easily curable), however, continue to cause about 9000 deaths a year. Because their primary cause, exposure to the sun, is so easily identified, these cancers are highly preventable. The most important preventive measure in controlling cancer is the reduction of tobacco use, which causes 30 percent of all deaths from cancer. Many more cancer cases can be prevented if individuals follow a healthy diet that includes reducing calorie intake to avoid obesity, reducing the number of calories derived from fat to 20 percent of the diet, and reducing the amount of red-meat consumed. Improving the diet also includes increasing the daily amounts of dietary fiber (whole grains, fruits, and vegetables) and protective foods (foods that contain vitamins C and A, as well as many vegetables, such as cabbage, cauliflower, broccoli, and brussel sprouts). Moderation in the use of alcohol and consumption of salt-cured, smoked, and nitrite-cured foods is advised. Also, avoid overexposure to sunlight, and routinely use sunscreens to prevent cancer of the skin. The environment can be improved by eliminating carcinogenic chemicals and asbestos fiber dust from workplace and home, and by reducing excess amounts of radon in buildings. Effective screening is available for cancer of the cervix, breast, colon, rectum, and prostate. Healthy individuals should have a cancer-related checkup every three years from age 20 to 40 and every year after age 40. Women past the age of 20 should conduct breast self-examinations every month and have a professional breast examination and pelvic examination yearly. A mammogram should be first performed between the ages of 35 and 39, and annually starting at age 40. Women who are sexually active or have reached the age of 18 should have a Pap test three years in a row; if each test result is negative for abnormal cell growth, women should discuss with a physician how often to have the test thereafter. A digital rectal exam should be performed every year after age 40 and a stool blood test every year after age 50. In men, a flexible proctoscopy should be done annually for two negative examinations and then every three to five years. Young men should conduct monthly self-examinations of the testes. Widespread adoption of these measures could virtually eliminate lung cancer, reduce the incidence of breast and colon cancer, and assure a high rate of cure for cancers of the breast, colon, rectum, cervix, and prostate. Contributed By: Charles Mason Huguley, Jr. Source: "Cancer (medicine)," Microsoft ® Encarta ® 98 Encyclopedia. © Microsoft Corporation. All rights reserved.
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Last modified: March 25, 2001 01:45 PM
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